In large-capacity power conversion devices, the converter output is high voltage or large current, and therefore, many large-capacity power conversion devices are configured with a plurality of converters multi-connected in series or parallel. Multi-connecting converters can not only increase the converter capacity, but also reduce harmonics contained in an output voltage waveform by synthesizing outputs, and as a result, can reduce harmonic current flowing to a grid.
As means for multi-connecting converters, there is a multilevel converter in which outputs of a plurality of converters are connected in cascade, and one of such multilevel converters is a modular multilevel converter. Each arm of the modular multilevel converter is composed of a plurality of converter cells connected in cascade.
Each of a first arm and a second arm for each phase of the conventional modular multilevel converter has a chopper cell (converter cell) and a reactor. The chopper cell has two semiconductor switches connected in series to each other, and a DC capacitor connected in parallel thereto. In each of the first arm and the second arm, the same number of chopper cells are connected in cascade via their respective output terminals.
Control for each phase of the conventional modular multilevel converter includes: average value control for causing the average value of voltage values of all DC capacitors to follow a capacitor voltage command value; individual balance control for causing the voltage value of each DC capacitor to follow the capacitor voltage command value; and arm balance control for causing the average value of voltage values of all the DC capacitors in the first arm and the average value of voltage values of all the DC capacitors in the second arm to coincide with each other. A voltage command value is calculated so as to control circulating current which circulates in the modular multilevel converter without flowing to the outside of the modular multilevel converter, and to control AC current for each phase (for example, see Patent Document 1 and Non-Patent Document 1).
In addition, in a power conversion device used for DC power transmission, a control device is often designed with respect to a specific operation point, and if an AC grid varies, the system might become unstable. In particular, in the case where a DC current path of a DC power transmission system is a long-distance cable, the inductance and the electrostatic capacitance of the cable increase and the resonance frequency of the DC current path decreases, and it is necessary to perform control that is stable against resonance of the DC circuit and ensures response performance. In the conventional control, DC voltage and DC current flowing through the DC current path are detected, the status value of the DC power transmission system is estimated from the detected DC voltage and DC current, and control for the DC voltage is performed, using the estimated status value, thereby suppressing resonance of the DC circuit (for example, see Patent Document 2).